The invention relates to an EUV (extreme ultraviolet) lithography apparatus, comprising: a housing enclosing an interior, at least one reflective optical element arranged in the interior, a vacuum generating unit for generating a residual gas atmosphere in the interior, and also a residual gas analyzer for detecting at least one contaminating substance in the residual gas atmosphere. The invention also relates to a method for detecting at least one contaminating substance by residual gas analysis of a residual gas atmosphere of an EUV lithography apparatus having a housing having an interior, in which at least one reflective optical element is arranged.
Reflective elements, in particular mirrors, are typically used as optical elements in EUV lithography apparatuses since there are no optical materials having a sufficiently high degree of transmission at the wavelengths used therein of approximately 13.5 nm. Operation of the mirrors under vacuum conditions, typically at (total) pressures of between 10−9 mbar and 10−1 mbar, is necessary in such projection exposure apparatuses because the lifetime of the multilayer mirrors is limited by contaminating substances. The contaminating substances are, inter alfa, heavy hydrocarbons in the gas phase, which are deposited on the mirror surfaces as solid carbon under irradiation with EUV light, whereby the reflectivity of the mirrors decreases.
The residual content of contaminating substances in the interior of the EUV lithography apparatus in which the optical elements are arranged should be determined as accurately as possible and in real time in order to define the point in time at which the exposure process in the EUV lithography apparatus can begin. At this point in time, the proportion of contaminating substances in the interior should lie below a predetermined limit value. The residual content of the contaminating substances can also be monitored during exposure operation, in order to be able to initiate suitable counter measures, if appropriate, in the event of an excessively large residual content, e.g. intensified pumping with a purge gas, in order that contaminating substances from the gas phase can deposit on the surfaces of the mirrors to the smallest possible extent.
In order to detect contaminations or contaminating substances in the gas phase, it is known from U.S. Pat. No. 7,212,273 B2 to use a quadrupole mass spectrometer to record a mass spectrum of the residual gas of an EUV lithography apparatus for gaseous substances between 40 and 200 atomic mass units. The ion current of the residual gas spectrum shown therein is not normalized, however, such that individual contaminating substances having extremely low partial pressures (<10−14 mbar) or individual ions of said contaminating substances cannot be detected by means of the mass spectrometer described therein.
WO 2008/034582 A2 in the name of the applicant discloses the use of a calibration leak for the calibration of a residual gas analyzer for an EUV lithography apparatus, by means of which leak a constant inflow of an inner gas into the interior takes place. Relative to the leakage rate of the calibration leak, the residual gas analyzer can be calibrated and its function can be checked. For a precise calibration it is expedient if the inert gas has a mass-charge ratio which is close to that of the contaminating substances to be detected. Particularly if a plurality of contaminating substances having significantly different mass-charge ratios are intended to be detected, it may be necessary, therefore, to use a plurality of different inert gases for the calibration.